The invention relates to a high-voltage X-ray tube, in which a cathode, held at negative high voltage during operating conditions, and an anode, held at positive high voltage during operating conditions, are disposed opposite each other in a vacuumized interior space. The invention relates in particular to such a high-voltage X-ray tube in which the anode is attached to an anode insulation element.
X-ray tubes in metal are preferably produced nowadays in coaxial design, the outer jacket of the tube being held at ground and the cathode or respectively the anode being fixed in the interior by a ceramic insulator. These specially shaped ceramic insulators are primarily made of Al2O3 ceramic. In addition, with these so-called metal-ceramic X-ray tubes, a distinction is made between two designs, depending upon the structure of the ceramic insulator, namely between the use of disc-shaped ceramic on the one hand, and the use of cylindrical or conical ceramic on the other hand.
In the case of the ceramic disc X-ray tube, the insulation path lies transversely to the tube axis. So that sufficient voltage sustaining capability can be ensured, the disc must therefore have an adequate diameter. A recommended value of 1 cm pro 50 kV thereby applies. The use of disc-shaped insulators thus leads to tubes with large diameter but short overall length. With such X-ray tubes, the high voltage is supplied by way of a special insulating adapter, which receives the cable with the high-voltage plug and connects it in a suitable way to the tube anode, or respectively tube cathode, via the ceramic disc element. The drawbacks of this solution consist in that the tube is given a large diameter and—owing to the lead covering necessary for reasons of radiation protection—a large amount of weight. Furthermore, for reasons relating to high voltage technology, the ceramic disc element is not ideal since the direction of field strength runs parallel to the ceramic surface, and thus there exists the risk of instability through flashover. Through the installation of a special adapter moreover two separate insulation bounding surfaces occur, which represent additional risks of high voltage flashover.
To avoid this and further drawbacks of the ceramic disc element, cylindrical or respectively conical insulations are also used. These have the electrical advantage that the field strength runs to a large extent perpendicular to the surface, and such a tube is therefore not as susceptible to electrical flashover. In addition, they make possible a narrow structural shape for the tube since the ceramic-insulation path runs parallel to the tube axis, and the dielectric strength is considerably greater through the vacuum than parallel to the ceramic surface. Since the commercially available high-voltage plugs can be directly connected to the ceramic element, a special adapter is not necessary, and the overall length can be kept short.
Since the anode in an X-ray tube is heated up by the electron beam during operation, a special anode cooling is required. This can take place in a simple way by air or water, but only in those situations in which the anode is held at ground potential, however. The X-ray tubes in which the one electrode, preferably the anode, is held at ground potential, and the second electrode, preferably the cathode, is held at high voltage are referred to as so-called unipolar X-ray tubes. Conversely, in so-called bipolar X-ray tubes, both electrodes are held at a high voltage.
As already mentioned a little further above, the conical ceramic as insulator in X-ray tubes has been especially successful. It permits, on the one hand, a high degree of voltage sustaining capability, and, on the other hand, it also ensures a compact design. In unipolar tubes, such a ceramic is preferably employed on the cathode side. The anode is then grounded, and can be cooled with water (or, if need be, with air).
A problem arises with bipolar tubes, however. Since the anode in these bipolar X-ray tubes is also held at high voltage, like the cathode, the anode cooling must be carried out in an insulated way since a high potential difference exists between the anode and the cooling unit. Insulating oil is preferably used in this case for cooling the anode. In this design, the integration of the anode cooling system in the conical ceramic part represents a problem, however. According to the conventional solutions, a special adapter must again be used for this, which contains the insulated cooling lines and the high voltage connection. This solution is complex and costly in manufacture; it also creates additional bounding surfaces at risk from high voltage, and leads moreover to an enlargement of the diameter of the tube. Thus the aims of simplicity in design, reliability and compactness of the X-ray tube cannot be achieved at all or only in insufficient measure.
It is desirable to propose a new high-voltage X-ray tube which does not have the drawbacks of the state of the art. It is desirable to provide such a bipolar high-voltage X-ray tube, which, on the one hand, has an especially compact design, and which at the same time also ensures the cooling of the anode, held at high voltage, such that reliability in operation is guaranteed at all times (in particular no oil leak possible, stable at high voltage), and that the tube is robust and is not damaged, even with transport and continuous operation.
According to an aspect of the invention a high-voltage X-ray tube is provided in which a cathode, which is held at negative high voltage during operating conditions, and an anode, which is held at positive high voltage during operating conditions, are disposed opposite each other in a vacuumized inner space, the anode being attached to an anode insulation element, the anode insulation element has a cylindrical shape or a shape tapering toward the anode, and includes an opening to receive a high-voltage plug, and a pipe structure is provided by means of which a coolant is able to be supplied to the anode. The advantage of this invention lies in particular in that the X-ray tube according to the invention is able to combine a compact design with a simple and reliable anode cooling. By means of the shape tapering toward the anode, all advantages of X-ray tubes can be kept which are based on the principle of the conical or cylindrical ceramic element. Moreover a significant reduction in size can be achieved, compared with conventional tubes, since the cooling of the anode can be achieved without additional adapter or other similar intermediate elements, thanks to the provided pipe structure.
In an embodiment variant, the coolant is insulating oil or another electrically non-conducting liquid or fluid. This embodiment variant has above all the advantage that already tried and proven means of cooling and approaches to cooling can be used. Furthermore, through the use of an electrically non-conducting liquid or fluid, the problem of unwanted electrical flashover is solved, so that the X-ray tube can be operated at all times with a high degree of safety and reliability.
In still another embodiment variant, the pipe structure comprises at least one inflow channel and at least one outflow channel. The advantage of this embodiment variant lies above all in that a closed circuit is formed, through which the coolant may circulate unhampered. By means of a channel structure in which the coolant flows in each case in one direction only, oil bottlenecks or other problems of this kind in the cooling system can moreover be completely eliminated. Finally, also a greater flow rate and thus also a faster cooling of the anode can thereby be ensured.
In a further embodiment variant, the pipe structure is integrated completely into the interior of the anode insulation element. The advantage of this embodiment variant lies in particular in that absolutely no additional elements are needed for anode cooling. The pipe structure is located completely inside the anode insulation element, so that in particular no changes in the high-voltage plug are necessary. In this embodiment variant, therefore, the standard high voltage plugs can continue to be used. Furthermore no special maintenance or special handling during operation is necessary since the pipe structure is well protected from external influences by the wall of the insulation layer.
In another embodiment variant, the pipe structure is integrated into the surface of the anode insulation element turned toward the high-voltage plug, the pipe structure being at least partially open toward the outside. This embodiment variant has in particular the advantage of very easy access to the pipe structure. In particular, this makes possible very simple control or respectively very simple upkeep of the pipe structure. Last but not least, the manufacture of such an anode insulation element is also considerably simplified. In addition, the maintenance is also further simplified since the grease otherwise used between the anode insulation element and the high-voltage plug resinifies, as a rule, over time and has to be renewed.
In still another embodiment variant, the pipe structure is able to be sealed off by the surface of the high-voltage plug. The advantage of this embodiment variant lies in particular in that the surface of the high-voltage plug can be used automatically for sealing off of the pipe structure. Thus also no additional elements or seals are needed. The coolant can then circulate freely in the respective channels, without oil leaks occurring.
In a further embodiment variant, provided on the high-voltage plug are sealing means, by means of which the pipe structure is able to be sealed off. This embodiment variant has above all the advantage that the sealing can be improved and optimized through use of special sealing means. Thus it can be ensured at all times that the pipe structure is closed off optimally, and that the coolant does not come into contact with the outer air. Moreover the high-voltage plug can also be pulled out even when the coolant is still in the pipe structure, for example, without this coolant escaping. The safety in use and the simplicity of handling of a high-voltage X-ray tube according to this embodiment variant are thereby brought to an especially high level. Likewise, in the case of damage to the sealing means, only this element has to be replaced, while the same high-voltage plug can continue to be used.
In still another embodiment variant, the pipe structure is integrated into the surface of the high-voltage plug. The most important advantage of this embodiment variant lies above all in the very easy access to the pipe structure, akin to the embodiment variant with the pipe structure in the surface of the anode insulation element. That makes possible above all very easy control or respectively very easy upkeep of the pipe structure, whereby the maintenance is also considerably simplified. However, made possible by means of this embodiment variant is the use of standardized anode insulation elements (i.e. such as are also usable with unipolar X-ray tubes, for example), while the pipe structure comes to lie exclusively within the high-voltage plug.
In another embodiment variant, the pipe structure is integrated into an intermediate element, the intermediate element being disposed between the anode insulation element and the high-voltage plug. The advantage of this embodiment variant lies in particular in that both standardized high-voltage plugs and standardized anode insulation elements are usable. Thus the pipe structure for guiding of the coolant is integrated into a completely new element, which can be manufactured and also installed separately. Also the maintenance of such a pipe structure is thereby especially simplified. In the case of failure or massive damage to the pipe structure, the intermediate element can also be very easily replaced, without the entire anode insulation element and/or the high-voltage plug being used also being affected.
In still another embodiment variant, the at least one inflow channel and/or the at least one outflow channel are of helical design. This embodiment variant has in particular the advantage that by means of the helical shape, a much greater length is achievable for the conducting channels. The insulation path in the coolant can thereby be increased, and the voltage sustaining capability and reliability of the tubes improved.
In a further embodiment variant, the at least one inflow channel and/or the at least one outflow channel are of rectilinear design. The advantage of this embodiment variant is in particular that a pipe structure with straight channels is produced considerably more easily and can essentially be maintained considerably more easily. In addition, the possibility of bottlenecks of liquid in the pipe structure is clearly smaller with straight channels. The coolant can also be conducted very quickly to the anode, which can be an advantage at very high temperatures.
In another embodiment variant, the pipe structure is producible by boring and/or casting. The advantage of this embodiment variant lies above all in that standardized manufacturing methods can be applied to produce the pipe structure for the coolant (e.g. the cooling liquid). A smooth surface of the channels in the pipe structure can also be obtained by boring or by casting, which is decisive for a friction-free flow of the cooling liquid.
In still another embodiment variant, the at least one inflow channel and/or the at least one outflow channel have a round or oval cross section. This embodiment variant has above all the advantage that an optimal channel shape is used for the circulation of the coolant in the pipe structure. Thus the possibility of a bottleneck or another difficulty is greatly reduced.
It should be mentioned here that in addition to the high-voltage X-ray tube according to the invention, this invention also relates to a corresponding method for producing the high-voltage X-ray tube according to the invention.